Effect of phase morphology on fatigue crack growth behavior of α-β titanium alloy—A crack closure rationale

Abstract

Effect of phase morphology on fatigue crack growth (FCG) resistance has been investigated in the case of an α-β titanium alloy. Fatigue crack growth tests with on-line crack closure measurements are performed in the microstructures varying in primary α (elongated/equiaxed/Widmanstatten) and matrix β (transformed/metastable) phase morphologies. The microstructures comprising metastable β matrix are observed to yield higher FCG resistance than those for transformed β matrix, irrespective of primary α phase morphology (equiaxed or elongated). But, the effect of primary α phase morphology is dictated by the type of β phase (transformed or metastable) matrix. It is observed that in the microstructures with metastable β matrix, the equiaxed primary α as second phase possesses higher FCG resistance as compared to that of elongated α morphology. The trend is reversed if the metastable β matrix is replaced by transformed β phase. The fatigue crack path profiles are observed to be highly faceted. The detailed fractographic investigations revealed that tortuosity is introduced as a result of cleavage in α or β or in both the phases, depending upon the microstructure. The crack closure concept has been invoked to rationalize the phase morphology effects on fatigue crack growth behavior. The roughness-induced and plasticity-induced crack closure appear to be the main mechanisms governing crack growth behavior in α-β titanium alloy.